The present invention relates to powdered graphite and a nonaqueous electrolyte secondary battery, and more particularly, relates to powdered graphite used for a negative electrode of a lithium ion secondary battery and a nonaqueous electrolyte secondary battery using the same.
In recent years, concomitant with the advancement of portable information apparatuses such as notebook personal computers and mobile phones, the demand of batteries has been rapidly increasing, and the application thereof also has been expanded. Under these circumstances, besides the trend toward miniaturization and reduction in weight of batteries, the increase in energy density of batteries has been demanded. In addition, from an energy conservation point of view, a high performance secondary battery which can repeatedly perform charge and discharge has also been desired.
As a secondary battery which can satisfy the demands described above, a lithium ion secondary battery has been widely used. Since obtaining a high energy density as compared to a lead secondary battery and a nickel-cadmium secondary battery, which are common aqueous electrolyte secondary batteries, the lithium ion secondary battery has been widely used, and intensive research and development attempting to realize higher performances has been carried out.
As a negative electrode active material used in a lithium ion secondary battery, a carbonaceous material such as non-graphitizable carbon or graphite has been used, and superior cycle properties have been obtained. Natural graphite has a scale shape, and due to its high crystallinity, a capacity close to the theoretical capacity (372 mAh/g) can be realized. Compared to a common capacity of artificial graphite in the range of 330 to 350 mAh/g, the capacity of natural graphite is 360 to 370 mAh/g, and hence a graphite negative electrode is most advantageously formed by filling natural graphite at a high volume density.
When artificial graphite is used, lithium ions are intercalated and deintercalated from various surfaces. On the other hand, as shown in FIG. 1, in natural graphite, carbon atoms form a network structure, and a number of AB planes (layer planes) spreading along the plane direction are laminated to each other to form a bulk shape. A surface formed by the layers thus laminated to each other is called an edge. When natural graphite is used for a negative electrode of a battery, lithium ions are intercalated from the edge in charging and are then diffused between the AB planes. In addition, in discharging, lithium ions are deintercalated and released from the edge.
An electrode used in a lithium ion secondary battery is generally composed of a metal thin film functioning as a collector and at least one active material layer provided on one side or both sides of the above metal thin-film. This active material layer is composed of carbon functioning as an active material, a conducting agent, and a binder binding the above materials to the collector, and the electrode is formed by the steps of applying an active material paste to the metal thin film to form an electrode plate, and drying the electrode plate, followed by appropriate rolling thereof. In this case, scale-shaped graphite having a high capacity tends to be oriented along the longitudinal direction of the electrode. In addition, the above graphite is very soft due to its high crystallinity, and the number of void portions present in the electrode is reduced since the filling properties of the graphite are improved by too much; hence, as a result, when the electrode is formed, an electrolyte may not permeate the electrode in some cases. In the case described above, intercalation/deintercalation reaction of lithium ions in the graphite occurs only at the surface of the electrode, and highly efficient charge and discharge may not be preferably performed.
Accordingly, in Japanese Unexamined Patent Application Publication Nos. 2003-197182 and 2003-197189, a method has been disclosed in which lithium ions are smoothly intercalated between layers of graphite and deintercalated therefrom by disposing the edges parallel to the electrode surface (scale-shaped graphite being disposed perpendicular to the electrode surface) as shown in FIG. 2 using the magnetic field orientation properties of the graphite.
However, when scale-shaped graphite is used for a negative electrode active material of a lithium ion secondary battery, the scale-shaped graphite tends to aggregate, and as a result, a problem may arise in some cases in that a uniformly blended paste is not obtained. The active material paste applied onto the collector for forming the negative electrode preferably has a viscosity of 3 to 12 Pa·s and a solid component content of 40% to 50%. However, in the case of a paste made from scale-shaped graphite, the graphite forms flocks (aggregates) at a solid component content of 40% to 50%, and as a result, application of the paste onto a metal foil may not be performed. In addition, when the solid component content of scale-shaped graphite is 35% or less, the viscosity is rapidly decreased to 1 Pa·s or less. In an electrode formed by applying the paste as described above, the active material layer is very liable to be peeled away, and as a result, the electrode thus prepared may not be easily used in practice.
In addition, when scale-shaped graphite is used without any modification thereof, since the area of the graphite is large, and a large amount of a binder adheres to the graphite, the binding between the active material layer and the metal is weak, and hence the active material layer is liable to be peeled away. Furthermore, when the amount of the binder is increased, the ratio of the active material is decreased, and as a result, the battery properties are degraded.